It is well-known to use high intensity, focused acoustic wave energy, such as ultrasonic waves (i.e., acoustic waves having a frequency greater than about 20 kilohertz) to generate thermal ablation energy for treating internal body tissue, such as tumors. It is also well-known to employ an imaging system, such as a MRI system, in order to guide the delivery of such high intensity ultrasound energy to the targeted tissue area, and to provide real-time feedback of the actual delivered thermal energy. One such image-guided, focused ultrasound system is the Exablate® 2000 system manufactured and distributed by InSightec Ltd, located in Haifa, Israel (www.Insightec.com).
By way of illustration, FIG. 1 is a simplified schematic representation of an image-guided, focused ultrasound system 100 used to generate and deliver a focused acoustic energy beam 112 to a targeted tissue mass 104 in a patient 110. The system 100 employs an ultrasound transducer 102 that is geometrically shaped and physically positioned relative to the patient 110 in order to focus the ultrasonic energy beam 112 at a three-dimensional focal zone located within the targeted tissue mass 104. The transducer 102 may be substantially rigid, semi-rigid, or substantially flexible, and can be made from a variety of materials, such as plastics, polymers, metals, and alloys. The transducer 102 can be manufactured as a single unit, or alternatively, be assembled from a plurality of components. While the illustrated transducer 102 has a “spherical cap” shape, a variety of other geometric transducer shapes and configurations may be employed to deliver a focused acoustic beam, including linear (planar) configurations. The ultrasound system 100 may further include a coupling membrane (not shown), such as an inflatable body or a balloon filled with degassed water, for providing or improving the acoustic coupling between the transducer 102 and the skin surface of the patient 110.
The transducer 102 may be formed of relatively large number of individually controlled elements 116 mounted on a distal (outward) facing surface 118 (best seen in FIG. 2) of the transducer 102. Each transducer element 116 may itself comprise one or more (adjacent) piezoelectric members electrically connected to a same drive signal supplied from a system controller 106. During operation, the individual piezoelectric members each contribute a fractional part of the ultrasound energy beam 112 by converting the respective electronic drive signal into mechanical motion and resulting wave energy. The wave energy transmitted from the individual piezoelectric members of the transducer elements 116 collectively forms the acoustic energy beam 112, as the respective waves converge at the focal zone in the targeted tissue mass 104. Within the focal zone, the wave energy of the beam 112 is absorbed (i.e., attenuated) by the tissue, thereby generating heat and raising the temperature of the tissue to a point where the cells are denatured (“ablated”).
An imager (e.g., an MRI system) 114 is used to generate three-dimensional images of the targeted tissue mass 104 before, during, and after the wave energy is delivered. The images are thermally sensitive so that the actual thermal dosing boundaries (i.e., the geometric boundaries and thermal gradients) of the ablated tissue may be monitored. The location, shape, and intensity of the focal zone of the acoustic beam 112 is determined, at least in part, by the physical arrangement of the transducer elements 116 and the physical positioning of the transducer 102. The location, shape, and intensity of the focal zone may also be controlled, at least in part, by controlling the respective output (e.g., phase and amplitude) of the individual transducer elements 116 by a process known as “electronic steering” of the beam 112. Examples of such physical positioning systems and techniques, and of electronic beam steering, including driving and controlling the output of individual transducer elements, can be found in U.S. Pat. Nos. 6,506,154, 6,506,171, 6,582,381, 6,613,004 and 6,618,620, which are all incorporated by reference herein.